1. Field of the Invention
The present invention relates to electrochemical catalyst particles, including nanoparticles, which can be used membrane electrode assemblies and in fuel cells.
2. Background of the Invention
Fuel cells are devices that convert the chemical energy of fuels, such as hydrogen and methanol, directly into electrical energy. The basic physical structure or building block of a fuel cell consists of an electrolyte layer in contact with a porous anode and cathode on either side. In a typical fuel cell, a fuel (e.g., methanol or hydrogen) is fed to an anode catalyst that converts the fuel molecules into protons (and carbon dioxide for methanol fuel cells), which pass through the proton exchange membrane to the cathode side of the cell. At the cathode catalyst, the protons (e.g., hydrogen atoms without an electron) react with the oxygen ions to form water. By connecting a conductive wire from the anode to the cathode side, the electrons stripped from fuel, hydrogen or methanol on the anode side, can travel to the cathode side and combine with oxygen to form oxygen ions, thus producing electricity. Fuel cells operating by electrochemical oxidation of hydrogen or methanol fuels at the anode and reduction of oxygen at the cathode are attractive power sources because of their high conversion efficiencies, low pollution, lightweight design, and high energy density.
In direct methanol fuel cells (DMFCs), the liquid methanol (CH3OH) is oxidized in the presence of water at the anode generating CO2, hydrogen ions and the electrons that travel through the external circuit as the electric output of the fuel cell. The hydrogen ions travel through the electrolyte and react with oxygen from the air and the electrons from the external circuit to form water at the anode completing the circuit.Anode Reaction: CH3OH+H2O→CO2+6H++6e−Cathode Reaction: 3/2O2+6H++6e−→3H2OOverall Cell Reaction:CH3OH+3/2O2→CO2+2H2O
Initially developed in the early 1990s, DMFCs were not embraced because of their low efficiency and power density, as well as other problems. Improvements in catalysts and other recent developments have increased power density 20-fold and the efficiency may eventually reach 40%. These cells have been tested in a temperature range from about 50° C.-120° C. This low operating temperature and no requirement for a fuel reformer make the DMFC an excellent candidate for very small to mid-sized applications, such as cellular phones, laptops, cameras and other consumer products, up to automobile power plants. One of the drawbacks of the DMFC is that the low-temperature oxidation of methanol to hydrogen ions and carbon dioxide requires a more active catalyst, which typically means a larger quantity of expensive platinum (and/or ruthenium) catalyst is required.
A DMFC typically requires the use of ruthenium (Ru) as a catalyst component because of its high carbon monoxide (CO) tolerance and reactivity. Ru disassociates water to create an oxygenated species that facilitates the oxygenation of CO, which is produced from the methanol, to CO2. Some existing DMFCs use nanometer-sized bimetallic Pt:Ru particles as the electro-oxidation catalyst because of the high surface area to volume ratio of the particles. The Pt/Ru nanoparticles are typically provided on a carbon support (e.g., carbon black, fullerene soot, or desulfurized carbon black) to yield a packed particle composite catalyst structure. Most commonly used techniques for creating the Pt:Ru carbon packed particle composite are the impregnation of a carbon support in a solution containing platinum and ruthenium chlorides followed by thermal reduction.
A multi-phase interface or contact is established among the fuel cell reactants, electrolyte, active Pt:Ru nanoparticles, and carbon support in the region of the porous electrode. The nature of this interface plays a critical role in the electrochemical performance of the fuel cell. Often, only a portion of catalyst particle sites in packed particle composites are utilized because other sites are either not accessible to the reactants, or not connected to the carbon support network (electron path) and/or electrolyte (proton path). In addition, there is a need for catalysts with increased activity, including Pt:Ru catalysts with improved properties for use in fuel cells.